Sylow p-Subgroups of Group of Order 2p
Theorem
Let $p$ be an odd prime.
Let $G$ be a group of order $2 p$.
Then $G$ has exactly one Sylow $p$-subgroup.
This Sylow $p$-subgroup is normal.
Proof 1
Let $n_p$ denote the number of Sylow $p$-subgroups of $G$.
From the Fourth Sylow Theorem:
- $n_p \equiv 1 \pmod p$
From the Fifth Sylow Theorem:
- $n_p \divides 2 p$
that is:
- $n_p \in \set {1, 2, p, 2 p}$
But $p$ and $2 p$ are congruent to $0$ modulo $p$
So:
- $n_p \notin \set {p, 2 p}$
Also we have that $p > 2$.
Hence:
- $2 \not \equiv 1 \pmod p$
and so it must be that $n_p = 1$.
It follows from Sylow $p$-Subgroup is Unique iff Normal that this Sylow $p$-subgroup is normal.
$\blacksquare$
Proof 2
Let $n_p$ denote the number of Sylow $p$-subgroups of $G$.
From the First Sylow Theorem, there exists at least $1$ Sylow $p$-subgroup of $G$.
Let $P$ be such a Sylow $p$-subgroup of $G$.
The index of $P$ is $2$.
From Subgroup of Index 2 is Normal, $P$ is normal in $G$.
From Sylow $p$-Subgroup is Unique iff Normal it follows that there is only $1$ Sylow $p$-subgroup of $G$.
$\blacksquare$
Proof 3
This is a specific instance of Group of Order $p q$ has Normal Sylow $p$-Subgroup, where $q = 2$.
$\blacksquare$